Light-permeable resins are widely used as materials for forming film-, plate-, and lens-like optical members for use in various optical devices (e.g., films, substrates, and prism sheets for use in liquid crystal display devices; lenses in lens systems for signal reading of optical disk devices; and Fresnel lenses and lenticular lenses for projection screens). Such resins are generally called “optical resins” or “optical polymers.”
Birefringence is one of important optical characteristics that should be taken into consideration when optical resins are used to form optical members. That is, in most cases, it is undesirable that optical resins have high birefringence. Particularly, in the above-mentioned applications (liquid crystal display devices, optical disk devices, projection screens, etc.), the presence of a birefringent film or lens in an optical path adversely affects image quality or signal reading performance, and therefore the use of an optical member made of an optical resin whose birefringence is as low as possible is desired. Needless to say, camera lenses, eyeglass lenses, and the like also preferably have low birefringence.
Meanwhile, as is well known in the art, birefringence of an optical polymer includes “orientation birefringence” mainly caused by the orientation of main chains of the polymer and “photoelastic birefringence” caused by stress. Orientation birefringence and photoelastic birefringence have their respective signs that depend on the chemical structure of a polymer, and are properties intrinsic to individual polymers.
That is, orientation birefringence generally develops when main chains of a chain-like polymer (polymer chains) are oriented, and this orientation of main chains occurs in a process involving the flow of a material, such as an extrusion molding or stretching process during production of a polymer film or an injection molding process frequently used for production of optical members having various shapes, and is fixed and remains in an optical member. Hereinafter, the phrase “orientation birefringence is positive” means that the refractive index is large in a direction parallel to the orientation direction of polymer chains, and the phrase “orientation birefringence is negative” means that the refractive index is large in a direction orthogonal to the orientation direction of polymer chains.
On the other hand, photoelastic birefringence is caused by elastic deformation (distortion) of a polymer. In the case of an optical member formed by using a polymer, elastic deformation (distortion) occurs and remains in its material due to, for example, volume contraction caused by cooling from a temperature around the glass transition temperature of the polymer to a temperature lower than that, and causes photoelastic birefringence. Further, the material is elastically deformed also by, for example, external force exerted on the optical member fixed to a device used at ordinary temperature (glass transition temperature or lower), which causes photoelastic birefringence. Hereinafter, the phrase “photoelastic birefringence is positive” means that the refractive index is large in a direction parallel to a direction in which tensile stress is applied (direction in which polymer chains are oriented), and the phrase “photoelastic birefringence is negative” means that the refractive index is large in a direction orthogonal to a direction in which tensile stress is applied.
So far, there are various studies about reports on the suppression of birefringence.
For example, PTL 1 discloses a non-birefringent optical resin material obtained by blending two polymer resins that are opposite in sign of orientation birefringence to each other and are completely compatible. However, it is difficult to uniformly mix the two polymer resins described in PR, 1 to obtain a practical polymer resin that uniformly exhibits low orientation birefringence as a whole, and aggregates of the polymer resins may cause defects due to foreign substances. Further, the polymer resins blended are different in their intrinsic refractive index, and therefore light scattering occurs due to non-uniformity of refractive index, which makes it impossible to obtain an optical material excellent in transparency. Although there is no description about photoelastic birefringence, it is conceivable that a polymer composition of an example will have significantly high photoelastic birefringence. Further, the mechanical strength, especially impact resistance, of the optical resin material is not always sufficient, which causes problems such as cracks when the optical resin material is practically used.
PTL 2 discloses a method for obtaining a non-birefringent optical resin material by adding, to a matrix formed of a transparent polymer resin, a low molecular substance whose orientation birefringence tends to cancel out the orientation birefringence of the polymer resin material. The low molecular substance has a molecular weight of 5000 or less, and a resulting molded body has excellent transparency. However, there is no description about improvement in photoelastic birefringence or mechanical strength. Further, there is a case where heat resistance is reduced.
PTL 3 discloses a method for obtaining an optical resin material having low orientation birefringence by adding, to a transparent polymer resin, a birefringent fine inorganic substance that is oriented in the same direction as the linked chains of the polymer resin as the polymer resin is oriented by the application of external force. Orientation birefringence can be suppressed also by this method, but there is no description about improvement in photoelastic birefringence or mechanical strength.
PTL 4 discloses a method for obtaining a non-birefringent optical resin material having low orientation birefringence and low photoelastic birefringence, in which an optical material having a multicomponent system of three or more components including a binary or higher copolymerization system is obtained by selecting the combination and constituent ratio (compositional ratio) of components of the multicomponent system so that both the orientation birefringence and photoelastic birefringence of the optical material are cancelled out at the same time. This method makes it possible to extremely reduce both orientation birefringence and photoelastic birefringence at the same time, which was unable to be achieved heretofore. However, the composition of the optical resin material is limited to some extent to make it possible to cancel out both orientation birefringence and photoelastic birefringence at the same time, and therefore the glass transition temperature of the optical resin material is as low as lower than 100° C., and there is a problem such as a reduction in mechanical strength. Further, there is also a problem that polymer decomposition occurs during molding perforated under such conditions that the optical resin material is retained at high temperature, such as melt-extrusion molding for forming a film.
In addition, in recent years, an acrylic resin film is expected to be developed for optical films as a resin film having relatively low birefringence. Reduction in weight and thickness of a film has rapidly advanced in displays, in particular, mobile displays, and further thinning is also required of an optical film used in such electronic devices. Therefore, with respect to the original film of an acrylic resin film, its thinning as well as improvement in mechanical strength by biaxial stretching has been considered. However, even after the biaxial stretching, the mechanical strength is not sufficient in some cases, and there are cases where film transport resistance, crack resistance at the time of actual use, and the occurrence of cracking or fine cracks in the trimming process at the time of film production or in the punching process of the device made by laminating the film cause a problem.
Then, for example, PTL 5 discloses a method for obtaining a resin composition, which has high heat resistance and exhibits excellent mechanical strength, especially bending resistance, when formed into a film, and an optical film. The resin composition and the optical film are obtained by using, in combination, an acrylic resin having a glass transition temperature of 120° C. or higher and a graft copolymer obtained by graft copolymerization of an acrylic rubber-like polymer and a vinyl group-polymerizable monomer (“core-shell”-type impact resistance improver, hereinafter also referred to as “core-shell polymer”). However, no data of orientation birefringence and photoelastic birefringence are shown in the examples, and therefore it is unclear whether the graft copolymer is effective at improving birefringence. Particularly, there is no description about improvement in photoelastic birefringence in the specification. PTL 5 states that the graft copolymer is added to improve mechanical strength. However, there is no description about the influence of the graft copolymer on birefringence and there is no description about the orientation birefringence and photoelastic birefringence in the examples, from which it is apparent that PTL 5 has no technical idea of imparting a function of adjusting birefringence to the graft copolymer.
PTL 6 discloses an optical film obtained by molding a resin composition containing an acrylic resin (A) and an acrylic rubber (B). The acrylic resin (A) is a heat-resistant acrylic resin (A-1) containing a repeating unit derived from a methacrylate monomer, a repeating unit derived from a vinyl aromatic monomer, a repeating unit derived from a methacrylate monomer having an aromatic group, and a cyclic acid anhydride repeating unit. This literature states that the optical film has high heat resistance and excellent trimming property and has excellent optical characteristics even when stretched. Although there is a description about improvement in trimming property, there is no description about the mechanical strength of the film other than trimming property, such as crack resistance on film bending, and therefore it is unclear from this literature whether the mechanical strength of the optical film is at such a level that the optical film can be practically used without problem. Further, optical films stretched 100% (stretched twice) in the examples have high birefringence (orientation birefringence), and none of the optical films of the examples is low in both orientation birefringence and photoelastic constant (photoelastic birefringence), and therefore improvement in birefringence is not sufficiently achieved. Further, as shown in the examples, the acrylic rubber (B) described in this literature is a so-called graft copolymer (core-shell polymer), and this literature states that the acrylic rubber (B) is added to improve mechanical strength while maintaining transparency such as haze. However, the influence of the acrylic rubber (B) on birefringence is not taken into consideration at all. For example, when a comparison is made between examples and comparative examples, the orientation birefringences of the optical films of examples to which the acrylic rubber (B) is added are adversely higher than those of optical films of comparative examples in which only the acrylic resin (A) is used, and the photoelastic constants (photoelastic birefringences) of the optical films of examples are equal to those of the optical films of comparative examples in which only the acrylic resin (A) is used. Further, the heat-resistant acrylic resin has a negative photoelastic constant, and the acrylic rubber (B) is also estimated to have a negative photoelastic constant from the composition thereof. Accordingly it is apparent that the acrylic rubber (B) deteriorates orientation birefringence and photoelastic birefringence, that is, this literature has no technical idea that the acrylic rubber (B) is used to adjust orientation birefringence and photoelastic birefringence.